Field of the Invention
The present invention relates to a micromechanical device having a single static electrode, and in particular to a micromechanical sensor for determining a power, acceleration or angular acceleration, and to a micromechanical actuator for the continuous or quasi-static deviation of light.
Description of Prior Art
A plurality of micromechanical actuators and sensors is based on the dependence of electrostatic capacities between static electrodes and an electrode on a moveable device on a deflection state of the device. The dependence of the one or the several capacities on the deflection state of a device may on the one hand be used in a sensor to detect the deflection state of a device using a capacity measurement. On the other hand the dependence of a capacity on the deflection state of the device causes that during the application of a voltage to the capacity a power operating on the moveable device influences its deflection state. This is the basis of micromechanical, electrostatically driven actuators.
In many cases, for fulfilling the functionality of an actuator or a sensor, respectively, a deflection of a device which is moveably attached or suspended, respectively, to a substrate via one or several springs or elastic connecting elements, respectively, wherein the deflection of the moveable device or body, respectively, needs to be possible at least in one degree of freedom or one dimension. To this end, opposed to the moveable device several electrodes are arranged, which are electrically contactable independent from each other, which are attached directly or indirectly to the substrate statically or fixed with respect to the substrate, respectively. To operate this micromechanical device as an actuator, one or several voltages are respectively applied on the same between a static electrode and the moveable device or an electrode, respectively. Depending on the fact between which static electrode and the moveable device an electric voltage is applied, the moveable device is deflected in the direction of the respective static electrode.
In order to use the described micromechanical device as a sensor, one or several electrostatic capacities are measured between one static electrode and the moveable device, respectively. A change of capacity between a certain static electrode and the movable device enables a direct conclusion to a deflection of the movable device in the direction of the static electrode towards the same or away from the same.
FIG. 5 is a schematical perspective illustration of a micromechanical device according to the prior art. The micromechanical device comprises a moveable member 2, which is attached to a frame 8 using a first elastic connecting element 4 and a second elastic connecting element 6. The moveable member 2 is typically substantially plate-shaped and integrally implemented with the elastic connecting elements 4, 6 and the frame 8 or parts of the frame 8. The frame 8 is attached on a surface 10 of a substrate 12. Although the moveable member 2, the elastic connecting elements 4, 6 and the frame 8 on the one hand and the substrate 12 on the other hand are illustrated in a spatially separated way, to be able to illustrate the surface 10 of the substrate 12, the frame 8 is attached to the surface 10 of the substrate 12 with a functional micromechanical device of the illustrated basic type. The frame only illustrates one possibility in an exemplary way, how the elastic connecting elements 4, 6 may be mechanically rigidly connected to the substrate 12. On the surface 10 of the substrate 12 two electrodes 14, 16 are further arranged opposite to the moveable member 2 which are electrically insulated from each other and from the moveable member 2.
The moveable member 2 and the elastic connecting elements 4, 6 comprise electrically conductive materials or are provided with a continuous electrically conductive layer on one surface. The substrate 12 typically comprises an electrically insulating material, wherein the electrodes 14, 16 are metal layers on the surface 10 of the substrate 12, or the substrate 12 comprises a semiconductor material, which is doped in the areas of the electrodes 14, 16 and is therefore electrically conductive and which is undoted and therefore electrically insulating outside the areas of the electrodes 14, 16.
The moveable member 2 comprises an inoperated position defined by the elastic connecting elements 4, 6, in which it is aligned in parallel to the surface 10 of the substrate 12 in the present example. The moveable member 2 may be deflected from this inoperated position by an external force or by an angular acceleration, wherein this deflection is bigger the stronger the force or the greater the acceleration, respectively.
A first electrostatic capacity between the first electrode 14 and the moveable member 2 and a second elect rostatic capacity between the second electrode 16 and the moveable member 2 are dependent on the deflection state of the moveable member 2. When the moveable member 2 tilts to the lefts with reference to the illustration in FIG. 5, it approaches the second electrode 16 and moves away from the first electrode 14, whereby the first capacity between the first electrode 14 and the moveable member 2 decreases and the second capacity between the second electrode 16 and the moveable member 2 increases. If the moveable member 2 contrarily tilts to the right with reference to the illustration in FIG. 5, the first capacity increases and the second capacity decreases.
Apart from an optimization of the micromechanical device principally illustrated in FIG. 5 for a concrete object or application, respectively, it may be operated both as a sensor and as an actuator. When the micromechanical device is operated as a sensor, the moveable member 2 is preferably suspended asymmetrically and the first capacity and the second capacity are measured by a not illustrated electronic evaluation means and from its measurement values the deflection state of the moveable member 2 is determined. From this, with a known elasticity of the connecting elements 4, 6, the acting force or acceleration may be determined.
When the micromechanical device is operated as an actuator, by a not illustrated means a first voltage between the first electrode 14 and the moveable member 2 and a second voltage between the second electrode 16 and the moveable member 2 is applied. These electric voltages generate forces which act on the areas of the moveable member 2 opposed to the respective electrode 14,16 and altogether result in a deflection of the moveable member 2 from its inoperated position. When, for example, the surface of the moveable member 2 facing away from the substrate 12 is implemented in a level and light-reflecting way, a deflection of the moveable member 2 caused by the two electric voltages may be used to reflect light, for example a laser beam, under a desired angle which may be set by the choice of the first and the second voltage. For manufacturing a micromechanical device, as it is schematically illustrated in FIG. 5, basically two methods are suitable, which are illustrated in the following.
With the hybrid construction, the first electrode 14 and the second electrode 16 are separated from the moveable member 2 as static counter-lectrodes, i.e. manufactured on a second work piece or wafer, respectively. In the article “Electrostatically actuated micromirror devices in silicon technology” by W. Lang et al. (Sensors and Actuators 74 (1999) 216-218) a member for deflecting light is described, wherein two static counter-electrodes which are controlled independent from each other are manufactured on a first wafer. The same is connected to a Pyrexplate by anodic bonding. A movable member suspended via springs or elastic connecting elements, respectively, is manufactured in a second wafer. Subsequently, the second wafer is also connected to the Pyrexplate by anodic bonding, so that the moveable member is opposed to the two static counter-electrodes. A corresponding construction is also described in the article “Electrostatically driven micromirrors for a miniaturized confocal laser scanning microscope” by U. Hofmann et al. (part of the “SPIE Conference on Miniaturized Systems with Micro-Optics and MEMS”, Santa Clara, September 1999, SPIE Vol. 3678, pp. 29-38).
In the article “Laser Display Technology” by J. Kränert et al. (“The eleventh Annual Intentional Workshop on Micro Electro Mechanical Systems”, Jan. 25th to 29th 1998, Heidelberg, IEEE catalogue No. 98CH36176, pp. 99-103) a micromirror array is described which comprises a glass wafer and a silicon wafer. Into the glass wafer cavities are etched into which electrodes are inserted to obtain a distance between the electrodes and the mirror plates. By wet-etching and chemically-mechanically polishing 15 μm thick membranes are produced. The mirror form is manufactured by plasma etching. The silicon wafer and the glass wafer are connected by anodic bonding. To this end, both in this and also in a further described method two static electrodes each are opposed to a micromirror.
U.S. Pat. No. 5,097,354 describes a beam sampling device or a beam scanner, respectively, having a light-emitting element and a moveable, reflective mirror. On the surface of one electrode base four static electrodes are formed. Opposed to the four coplanar electrodes the mirror is connected to a mirror bases via torsion bars, which are rigidly mounted to the electrode basis using pins.
U.S. Pat. No. 4,317,611 describes a light-beam deflection device of the torsion-type, which comprises two etched plates. One of the two comprises a monocrystalline semiconductor material, like e.g. silicon, the other plate comprises a suitable insulating material, like e.g. glass. The semiconductor plate is etched to form an oblong bar having a broader middle section which comprises a reflective surface. In the middle of the insulating plate a depression is etched, wherein an oblong ridge may come to lie in the middle of the insulating plate under the area comprising the reflective surface and the torsion bar, to support the same in the direction perpendicular to its longitudinal axis, wherein its rotation around the longitudinal axis is enabled. In the depression within the insulating plate level electrodes are disposed which are provided to cause an electrostatic force between one of the level electrodes and the semiconductor device and therefore cause an angular deflection around the longitudinal axis of the torsion bars.
U.S. Pat. No. 5,629,790 describes micro-processed torsion scanners wherein two electrodes are respectively opposed to one mirror held by torsion bars in the different proposed geometries. A silicon member comprising the mirror and a dielectric substrate comprising the two electrodes are manufactured separately.
One advantage of the hybrid construction is that it does not comprise an inherent limitation of the distance between the moveable member and the static counterelectrodes, so that also great distances are possible and consequently great deflections of the moveable member may be generated or detected, respectively.
One disadvantage of the hybrid construction is the cost- and time-consuming manufacturing process which is based on the use and processing of several wafers. As the static counter-electrodes manufactured on a wafer must be aligned with a high precision with respect to the moveable members manufactured on another wafer and opposed to the elastic connecting elements, the two wafers must be aligned before a bonding process with just this high accuracy, from which a substantial manufacturing effort results. Apart from that, the construction and connection technology is cost- and time-consuming, as the static counter-electrodes and the moveable member are not arranged in a coplanar way and therefore also their electrical contacting is firstly required in two different levels. Also a contacting of the static counter-electrodes via the substrate or the member backside, respectively, is not easily possible, as two electrically independent electrodes need to be contacted.
In an integrated construction using a so-called sacrificial layer the member suspended moveable via elastic connecting elements and the static counterelectrodes are manufactured on one single substrate. To this end, first of all the static counter-lectrodes are manufactured on the substrate. Then, the sacrificial layer is applied to the substrate containing the static counter-electrodes. On the sacrificial layer a layer of an electrically conductive material is applied and structured in the desired form of the moveable member and the elastic connecting elements. Subsequently, the sacrificial layer is removed and therefore the moveable member and the elastic connecting elements are revealed.
The article “Novel beam steering micromirror device” by R. W. Fuchs et al. (part of the “SPIE Conference on Miniaturized Systems with Micro-Optics and MEMS”, Santa Clara, September 1999, SPIE Vol. 3878, pp. 40-49) describes a micromirror device for light modulation, wherein an electrically conductive mirror plate is arranged substantially in parallel to a substrate having several static electrodes and connected to the substrate via an elastic joint structure laterally centrically arranged between the mirror plate and the substrate. The static electrodes, the elastic joint structure and the mirror plate are subsequently manufactured directly on the substrate and using a sacrificial layer which is subsequently removed.
The article “Design and fabrication of micromirror array with hidden joint structures” by C.-H. Ji and Y.-K Kim (part of the “SPIE Conference on Miniaturized Systems with Micro-Optics and MEMS”, Santa Clara, September 1999, SPIE Vol. 3878, pp. 71-77) describes the design and the manufacture of an array of micromirrors having microprocessed surfaces. A mirror plate is substantially arranged in parallel to a substrate comprising several electrodes and is spaced apart from the same. The mirror plate and substrate are moveably connected to each other by a connecting structure centrically arranged between the two, wherein the connecting structure comprises a pin and clamps, and wherein a movement of the mirror plate around an axis defined by the pin is enabled.
The article “Micromirrors for direct writing systems and scanners” by H. Lakner et al. (part of the “SPIE Conference on Miniaturized Systems with Micro-Optics and MEMS”, Santa Clara, September 1999, SPIE Vol. 3878, pp. 217-227) describes cantilever beam micromirrors which include stand-alone mirror elements which are held by support pillars via an air gap and a lower address electrode.
In the integrated construction the sacrificial layer is removed either wet-chemically or using a reactive gas. With dimensions of the moveable member of more than several micrometers additional openings in the moveable member are required to enable the removal of the sacrificial layer. These openings reduce the optical quality of the surface of the moveable member which restricts its usage as an electrostatically controllable mirror in optical applications. Also the thickness of the sacrificial layer is restricted for reasons of the planarity and the production of pillars which illustrate the mechanical connection of the elastic connecting elements to the substrate. Thus, the maximum deflection of the moveable member is restricted. For some applications it is decisive, however, to enable a large change of the electrical capacity between one stationary electrode each and the moveable member for generating or detecting, respectively, a deflection of the moveable member and simultaneously enable a large maximum deflection of the moveable member. This requirement is for example present in electrostatically driven scanners. As the thickness of the sacrificial layer is restricted to few micrometers, however, the deflection angle in the case of a moveable member with large dimensions, i.e. dimensions in the range of several micrometers, is so small that the use of such members is strongly restricted.
In a moveable member described in the above-mentioned article “Novel beam steering micromirror device” having dimensions of 300 μm×300 μm, due to the restriction of the thickness of the sacrificial layer to several micrometers a deflection angle of only two degrees is obtainable. The field of the use of the device is therefore strongly restricted. The same applies to the device described by Ji and Kim. The micromechanical devices described by Lakner et al. either comprise only one single pair of opposing electrodes, which is the reason why the moveable member may be deflected from the inoperated position only in one direction, or only comprise electrodes, which drive the moveable member into the inoperated position and are therefore only useable as an actuator which oscillates using its inherent frequency. To this end, in the production of the device described by Lakner et al. no sacrificial layer is used.
U.S. 6,188,504 B1 describes an optical scanner comprising a support member for mounting on a given member, a moveable plate with a reflective surface for reflecting light, and an elastic member that couples the moveable plate and the support member to each other. The elastic member comprises a plurality of laminated organic elastic insulating layers. An actuator is at least provided on the moveable plate for generating a driving force between the moveable plate and the support member. An electrical element is provided to apply a predetermined electrical signal to the actuator and thus generate the driving force, whereby the elastic member is elastically deformed and the moveable plate is deflected. The electrical element is provided between the organic elastic insulating layers of the elastic member. According to one embodiment, the actuator is an electrostatical actuator, wherein two moveable electrodes are provided on the surface of the moveable plate and one stationary electrode is provided on a fixed member.